1. Field of the Invention
The present invention relates to a semiconductor device including a voltage conversion circuit which converts the voltage value of an input voltage according to the change of the resistance value of a resistance section provided in a circuit as temperature changes and which outputs an output voltage.
2. Description of the Background Art
Recently, as a portable device such as a cellular phone and a PDA (Personal Digital Assistant) spread widely, the quantities of information processed by these devices rapidly increase. Therefore, the capacity of a work memory is greatly increased and a DRAM (Dynamic Random Access Memory) cell-based semiconductor memory is mainly employed as the work memory.
For the portable device driven by a battery, however, the increase of standby current due to a refresh operation which is essential to the DRAM cell is considerably problematic.
Meanwhile, the data holding time of a DRAM cell is greatly influenced by operating temperature. As the operating temperature rises, the data holding time is shortened. It is therefore desirable to change the refresh interval of the DRAM cell as well as the temperature thereof so as to conform to this characteristic.
Specifically, by controlling the refresh interval to become wider as temperature is lower, the refresh operation current of the DRAM cell at a temperature near a room temperature, at which the portable device is normally used, is lowered and the current consumption of the portable device in a standby state is decreased.
Japanese Patent Laying-Open No. 9-204773 discloses a method for adjusting a current carried to a ring oscillator which serves as an oscillation circuit for setting a refresh interval according to temperature.
Conventionally, a voltage conversion circuit which supplies a constant voltage to the gate electrode of a transistor in the next stage so that the transistor in the next stage can carry a constant current, i.e., a circuit (hereinafter, referred to as “voltage conversion circuit”) which functionally converts the voltage value of an input voltage into a different voltage value and which outputs an output voltage has temperature dependency and changes the output voltage.
A current carried to the ring oscillator changes according to the change of this output voltage and the oscillation frequency of the ring oscillator changes, accordingly.
However, it is known that the temperature dependency of the voltage conversion circuit disadvantageously becomes irregular due to the manufacturing irregularities such as the manufacturing irregularity of the resistance value of the resistance connected between a MOS transistor and a power supply voltage line.
The irregular temperature dependency does not always occur in the same manner per wafer or per chip in a wafer plane.